The present invention relates to an electrophotographic color image forming apparatus and, more particularly, to a color image forming apparatus for forming a plurality of single-color images on a latent image carrier and sequentially registering the images to form a color image.
An electrophotographic color image forming apparatus which forms an electrostatic latent image on a photosensitive drum, forms a visible image from the latent image using toners, and transfers the image to recording paper, uses various schemes. For example, the following schemes are used. (1) A process of forming, on a photosensitive drum, a toner image of one of three primary colors (yellow, magenta, and cyan) for forming a color image, and transferring the toner image to recording paper is repeated three times in correspondence with the colors (or four times, including black, as needed). (2) Image forming units for forming single-color toner images are arranged in the recording paper convey direction in correspondence with the three primary colors and black. Single-color images are sequentially registered on recording paper to form a color image. (3) Image forming units are arranged in the photosensitive drum moving direction in correspondence with the primary colors. Single-color images are sequentially registered on the surface of the photosensitive drum to form a color image and transferred to recording paper at once. Scheme (3) includes dry electrophotography using dry toners and wet electrophotography using liquid developers containing fine toners.
In the above-described schemes, to form a satisfactory color image, single-color images must be accurately registered. If this registration accuracy is not sufficiently high, the image quality degrades: image blur or a shortage of sharpness or color reproducibility occurs. A registration accuracy at which the registration error is not visually recognized and a high image quality is obtained is generally 0.1 mm or less.
Conventionally, various proposals have been made in order to improve the registration accuracy. For example, in a color image forming apparatus with a tandem arrangement, marks for detecting the registration errors of single-color images are formed on the recording paper conveyor belt, and the image formation position is corrected on the basis of detected registration error data.
In this technique, however, toner consumption increases because of formation of registration error detection marks, a cleaning unit for removing unnecessary marks that remain after detection and are not associated with the output image must be added, the photosensitive material degrades due to mechanical stress in cleaning, the recording time is prolonged by the loss time for registration error detection, and cost increases because of the necessity of a detector for accurately detecting the marks. Additionally, running cost increases, including the cost of photosensitive material and toner.
Assume that a detector for accurately detecting a registration error detection mark formed from a toner pixel with a dot size associated with the recording resolution is realized to detect the registration errors of the single-color images. When the allowable registration error amount is about 0.1 mm, the detection accuracy of the detector itself must be increased to 0.02 mm or less in consideration of the accuracy allotment by analysis of registration error generating factors and analysis of errors in constituent elements of the image forming apparatus. In addition, since the image forming process for forming marks or constituent element driving accuracy varies, the detection accuracy must be increased by detecting a plurality of marks and statistically processing them. This increases not only load on hardware including the detector but also load on software for the detection sequence.
There is a technique of accurately registering images by multiple-exposure in which one of a plurality of exposure laser beams is used as a reference, and the remaining beams are synchronized with the reference beam. In this technique, however, measures against an image registration error generated by a shift in beam image formation position due to a thermal deformation of an optical system element according to a change in ambient temperature or an image position shift due to a shift in the relative positions of each laser beam and the photosensitive drum in exchanging the photosensitive drum are not particularly considered.
When the photosensitive drum driving speed has a slight error, an image registration error occurs due to this speed error. In addition, since a mechanism for attaching, at a sufficiently high accuracy, a photosensitive material which is expendables and must be exchanged several ten times during the service life of the apparatus is required, the apparatus arrangement becomes complex. Furthermore, the photosensitive material driving speed need also be accurately set. Hence, a drive transmission mechanism with high mechanical accuracy and photosensitive driving by accurate speed control are necessary, resulting in an increase in cost of the apparatus.
There is a technique of decreasing the positional shift between color toner images to increase the registration accuracy, in which support members are provided around the photosensitive drum to increase the positioning accuracy of the image exposure devices. However, along with the recent requirement for a high-speed high-resolution color image forming apparatus, strong demand has arisen for improvement of the registration accuracy of color toner images. No sufficient registration accuracy is obtained only with the positioning accuracy of units.
Accordingly, it is an object of the present invention to provide a color image forming apparatus capable of accurately correcting a registration error in color images and forming a high-quality color image at a high speed.
According to one aspect of the present invention, there is provided a color image forming apparatus comprising: a latent image carrier; an image exposure mechanism configured to scan a surface of the latent image carrier with exposure light beams in accordance with image information of different colors to form a plurality of latent images; a development mechanism configured to develop the plurality of latent images formed by the image exposure mechanism into visible images to form a color image on the latent image carrier; at least one reflector provided on the latent image carrier; at least one detector for detecting reflected light beams of the exposure light beams from the at least one reflector and outputting a detection signal; and a correction mechanism configured to control relative positions of the plurality of latent images to be formed by the image exposure mechanism, on the basis of the detection signal from the at least one detector.
According to another aspect of the present invention, there is provided a color image forming apparatus comprising: a latent image carrier; an image exposure mechanism configured to scan a surface of the latent image carrier with exposure light beams in accordance with image information of different colors to form a plurality of latent images; a development mechanism configured to develop the plurality of latent images formed by the image exposure mechanism into visible images to form a color image on the latent image carrier; a reflector provided on the latent image carrier, the reflector having a plurality of reflection members; a detector for detecting reflected light beams of the exposure light beams from at least one of the plurality of reflection members and outputting a detection signal; and a correction mechanism configured to control relative positions of the plurality of latent images to be formed by the image exposure mechanism, on the basis of the detection signal from the detector.
According to still another aspect of the present invention, there is provided a color image forming apparatus comprising: a latent image carrier; a plurality of image exposure units for forming, on a surface of the latent image carrier, latent images of exposure light beams in accordance with image information of different colors by exposure; a plurality of development units for developing the latent images formed by the plurality of image exposure units into visible images to form a color image on the latent image carrier; a detection pattern formation mechanism configured to form, on the latent image carrier using at least one of the plurality of image exposure units, a plurality of detection patterns used to detect at least one of a relative difference and a tilt angle of each of the visible image formation positions of the plurality of development units; a detector for detecting the-plurality of detection patterns; and a correction mechanism configured to obtain at least one of the relative difference and the tilt angle on the basis of a detection result from the detector and control relative positions of the latent images to be formed by the plurality of image exposure units, on the basis of the obtained relative difference or tilt angle.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.